This invention relates to a continuous process for the production of allophanate-modified diphenylmethane diisocyanates having an NCO group content of 19 to 32% by weight and an urethane content of less than 2 area % as determined by GPC analysis. The process comprises (1) continuously reacting (a) diphenylmethane diisocyanate and (b) an alcohol, in the presence of (c) at least 25 ppm of an allophanate catalyst, based on the combined weight of the diphenylmethane diisocyanate and the alcohol, in a reactor in an oxygen-free environment; (2) continuously treating the product exiting the reactor with a catalyst stopper at the reaction temperature; and (3) cooling the resultant product. The allophanate catalyst can be dissolved in either the diphenylmethane diisocyanate or in the alcohol.
Allophanate-modified di- and poly-isocyanates are known and described in, for example, U.S. Pat. Nos. 4,160,080, 4,738,991, 4,866,103, 5,319,053 and GB 994,890.
U.S. Pat. No. 4,160,080 discloses a process for the preparation of allophanates which containing aliphatically and/or cycloaliphatically bound isocyanate groups in which compounds containing urethane groups are reacted with polyisocyanates having aliphatic and/or cycloaliphatic isocyanate groups, in the presence of a strong acid. The process is generally conducted at a temperature of from 90 to 140xc2x0 C. for about 4 to about 20 hours. All of the working examples describe a batch process.
Storage-stable polyisocyanates having allophanate linkages are disclosed by U.S. Pat. No. 4,738,991. These polyisocyanates containing allophanate linkages are prepared by reacting an organic polyisocyanate with a mono- or polyhydric compound in the presence of an organo-metallic catalyst. The catalyst is then deactivated by a compound such as an inorganic acid, an organic acid, an organic chloroformate or an organic acid chloride. Only a batch process is described. All of the examples use toluene diisocyanate with ethylene glycol to form the polyisocyanates having allophanate linkages.
Polyisocyanate compositions are disclosed in U.S. Pat. No. 4,8661,103. These polyisocyanates comprise the reaction product of an alcohol or thiol having an average functionality of from about 1.5 to about 4 and an average equivalent weight of at least 500 with at least 2 equivalents per hydroxyl and/or thiol equivalent of an organic poly-isocyanate (including the 4,4xe2x80x2- and 2,4xe2x80x2- isomers of diphenylmethane diisocyanate) under conditions such that at least about 20% of the initially formed urethane and/or thiourethane groups are converted to allophanate and/or thioallophanate groups. The only working example illustrating the preparation of an allophanate modified isocyanate uses a batch process.
U.S. Pat. No. 5,319,053 discloses stable, liquid, allophanate-modified diphenylmethane diisocyanates having NCO group contents of 12 to 32.5% by weight, and prepolymers of these stable, liquid, allophanate-modified diphenylmethane diisocyanates. Batch processes for the production of these products are also disclosed. The allophanate-modified diphenylmethane diisocyanates of this reference may be prepared by (1) pre-reacting the diphenylmethane diisocyanate with an aliphatic alcohol to form a urethane, which is subsequently converted to an allophanate; or (2) reacting the aliphatic alcohol, diphenylmethane diisocyanate and catalyst to form the allophanate directly. Although the batch process described therein has been used successfully in commercial operations, it is desirable to produce substantially identical products via a continuous process due to lower costs, resulting from smaller reactors having substantially higher throughput.
Allophanate modified polyisocyanates are also disclosed in GB 994,890. These are obtained by reacting an amount in excess of n moles of an organic diisocyanate with one mole of a urethane isocyanate of the specified formulation, with the reaction being carried out under conditions such that substantially one molecule of diisocyanate reacts with each urethane group present, as indicated by the measured isocyanate group content of the reaction mixture. Suitable conditions for the reaction include heat alone, or in the presence of a catalyst such as, for example, a metal carboxylate, a metal chelate or a tertiary amine. Only batch processes are described for the preparation of allophanate-modified isocyanates.
Advantages of the present invention include a novel method of preparing, at various NCO group contents, a consistent allophanate product at a lower cost from MDI and alcohols in the presence of an allophanate catalyst using inexpensive equipment. The present invention also describes the most probable method for introduction of the allophanate catalyst and the limitations of using MDI as a vehicle to deliver the allophanate catalyst.
This invention relates to a continuous process for the production of allophanate modified diphenylmethane diisocyanates having NCO group contents of from about 19 to about 32% by weight, and having a urethane content of less than 2 area % by GPC analysis. This process comprises:
(1) continuously reacting
(a) diphenylmethane diisocyanate comprising
(i) from about 0 to about 60% by weight of 2,4xe2x80x2-diphenylmethane diisocyanate,
(ii) less than about 6% by weight of 2,2xe2x80x2-diphenylmethane diisocyanate, and
(iii) the balance being 4,4xe2x80x2-diphenylmethane diisocyanate, with the sum of the %""s of (a)(i), (a)(ii) and (a)(iii) totaling 100% by weight of (a), the diphenylmethane diisocyanate; and
(b) an alcohol; in the presence of
(c) at least 25 ppm of an allophanate catalyst, based on the combined weight of the diphenylmethane diisocyanate and the alcohol;
in at least one reactor at a temperature of from 80 to 110xc2x0 C., preferably 90 to 100xc2x0 C., most preferably about 90xc2x0 C., for about 0.5 to 4 hours, preferably 1 to 2 hours and most preferably about 1 to about 1.5 hours, in an oxygen free environment (preferably in the presence of an inert gas such as, for example, nitrogen;
(2) continuously treating the product exiting the reactor with a catalyst stopper, with the stopper being present in an amount such that there is at least 1 mole of stopper for each mole of catalyst and more preferably from 1 mole to 4 moles of stopper for each mole of catalyst, and the stopper being added at the reaction temperature; and
(3) cooling the resultant product, preferably to a temperature of from about 25 to about 30xc2x0 C.
The continuous process of the present invention can be performed, for example, in at least one reactor, wherein the reactants are continuously fed into the reactor and the product continuously exits the reactor. It is preferred to use either a plug-flow reactor, or a cascade overflow reactor system. In cascade-overflow reactor system, it is preferred that the system comprise at least two (2) reactors, more preferably from two (2) to four (4) reactors, and most preferably three (3) reactors.
Suitable reaction temperatures for the first step, i.e. continuously reacting diphenylmethane diisocyanate with an alcohol, in the present process are from about 80 to about 110xc2x0 C., preferably from about 90 to about 100xc2x0 C. and most preferably about 90xc2x0 C., for time periods of from about 0.5 to about 4 hours, preferably about 1 to about 2 hours, and most preferably about 1 to about 1.5 hours. These residence times represent the total reaction time for all reactors present.
Suitable reactor systems and feed systems for the present continuous process have oxygen-free environments. It is preferred that the entire system including feed systems as well as the reactors are purged with an inert gas. Some examples of inert gases suitable for this purpose include compounds such as nitrogen, helium, neon, argon, etc. Nitrogen is a particularly preferred inert gas for the present invention.
Suitable (a) diphenylmethane diisocyanates for the present process include those which comprise:
(i) from about 0 to about 60% by weight, preferably from about 1 to about 3% by weight, and most preferably from about 1 to about 2% by weight of the 2,4xe2x80x2-isomer of diphenylmethane diisocyanate;
(ii) less than about 6% by weight, preferably from about 0 to about 1% by weight, and most preferably from about 0 to about 0.2% by weight of the 2,2xe2x80x2-isomer of diphenylmethane diisocyanate; and
(iii) the balance being 4,4xe2x80x2-diphenylmethane diisocyanate; with the sum of the %""s by weight of (a)(i), (a)(ii) and (a)(iii) totaling 100% by weight of (a) the diphenylmethane diisocyanate component.
It is preferred that diphenylmethane diisocyanates suitable for the present invention have an acidity of less than 10 ppm (as HCI), more preferably less than 5 ppm and most preferably less than 3 ppm.
Suitable alcohols for component (b) of the present invention include aliphatic alcohols and aromatic alcohols. Some examples of suitable aliphatic alcohols include those having from 1 to 36 carbon atoms, preferably from 4 to 16 carbon atoms, and most preferably 4 to 8 carbon atoms. Illustrative but non-limiting examples of these aliphatic alcohol""s can be selected from the group consisting of cycloaliphatic alcohol""s, aliphatic alcohols containing aromatic groups, aliphatic alcohols containing groups that do not react with isocyanates, e.g. ether groups and halogens such as, for example, chlorine and bromine. Other specific examples of some suitable aliphatic alcohols include compounds such as 2-butanol, cetylalcohol, cyclohexanol, 2-methoxyethanol, 2-bromoethanol, isobutyl alcohol, isooctyl alcohol, etc. A particularly preferred aliphatic alcohol is isobutyl alcohol.
Examples of suitable aromatic alcohols are those compounds containing 6 to 18 carbon atoms, preferably 6 to 12 carbon atoms, Wherein the hydroxyl group is directly attached to the aromatic ring. Some suitable aromatic alcohols include, for example, phenol, 1-naphthol, and substituted phenols such as cresol and substituted naphthols such as 3-methyl-1-naphthol. Preferred aromatic alcohols are phenol and the. substituted phenols.
The present invention also requires a suitable allophanate catalyst. Some examples of these catalysts include zinc acetylacetonate, zinc 2-ethylhexanoate, cobalt 2-ethylhexanoate, cobalt naphthenate, lead linoresinate, etc. Zinc acetylacetonate is a preferred catalyst.
In accordance with the present invention, the allophanate catalyst is present in a sufficient amount such that there is at least 25 ppm of catalyst, based on the combined weight of the diphenylmethane diisocyanate and the alcohol components. It is preferred that there are at least 40 ppm of catalyst, and most preferably from 40 to 75 ppm of catalyst present, based on the combined weight of the diphenylmethane diisocyanate and the alcohol components.
Once the product leaves the reactor (or the last reactor if a cascade-overflow reactor system is being used), the product should be maintained at the same temperature, or close to it, until after the addition of a catalyst stopper. This is because if the product is allowed to cool before the stopper is added, it leads to the promotion of polymeric allophanate species having a functionality greater than 2.0.
Suitable catalyst stoppers for the present invention include those which are generally known to be effective stoppers for batch processes to form allophanate-modified isocyanates. Some examples include acidic materials such as anhydrous hydrochloride acid, sulfuric acid, bis(2-ethylhexyl)hydrogen phosphate, benzoyl chloride, Lewis acids, etc. Benzoyl chloride is the preferred stopper. The amount of stopper to be added is generally such that there is at least 1 mole of stopper present for each mole of catalyst present, and more preferably from 1 mole to 4 moles of stopper for each mole of catalyst present.
Once the addition of-the catalyst stopper is complete, preferably a few minutes after addition of the stopper, the resultant product is cooled. It is preferred that the product is cooled to a temperature between 25 and 30xc2x0 C.
In accordance with the present invention, the allophanate catalyst may be dissolved in the diphenylmethane diisocyanate component and introduced into the reactor as a mixture, with the alcohol component being added separately, or the allophanate catalyst may be dissolved in the alcohol component and introduced into the reactor as a mixture, with the diphenylmethane diisocyanate component being added separately. It is also possible that some allophanate catalyst be dissolved in each of the two components, i.e. the alcohol component and the diphenylmethane diisocyanate component. The amount of allophanate catalyst added to either or both components should be such that there is at least 25 ppm of allophanate catalyst present, based on the combined weight of the diphenylmethane diisocyanate component and the alcohol component.
When the allophanate catalyst is added to the diphenylmethane diisocyanate component, the mixture should be stored at temperatures of no more than 60xc2x0 C., preferably from 40 to 50xc2x0 C. and most preferably from 40 to 42xc2x0 C. In addition, the mixture of diphenylmethane diisocyanate should be used within 20 hours of when it is prepared, and preferably within 8 hours of being prepared. It has been found that if this mixture of MDI-allophanate catalyst is used after 20 hours, the color of the resultant allophanate-modified diphenylmethane diisocyanates increased and was darker, with slightly higher viscosities due to the formation of side products (i.e. dimers and/or trimers).
In the present invention, it is preferred that the allophanate catalyst is added to the alcohol component. In this embodiment, the mixture should be stored at temperatures of no more than 50xc2x0 C., preferably from 20 to 40xc2x0 C. and most preferably from 20 to 30xc2x0 C. In addition, the mixture of alcohol component and allophanate catalyst, when stored at temperatures between 45 and 50xc2x0 C., should be used within 3 weeks of when it is prepared, and preferably within 2 weeks of being prepared. It has been found that if this mixture of alcohol-allophanate catalyst is used after 3 weeks, the catalyst loses some of its reactivity and could lead to urethane not being completely converted to allophanate (i.e.  greater than 2 area % by GPC). In addition, the mixture of alcohol component and allophanate catalyst, when stored at temperatures  less than 40xc2x0 C., should be used within 3 months of when it is prepared. This embodiment allows more flexibility in terms of length of time a mixture of alcohol-catalyst can be stored prior to being used in the presently claimed process and temperatures at which these mixtures can be stored and/or used at, without adversely affecting the allophanate-modified diphenylmethane diisocyanates produced by this continuous process.
The allophanate-modified diphenylmethane diisocyanates produced by the presently claimed continuous process have essentially identical NCO 1group contents as those allophanate-modified diphenylmethane diisocyanates prepared by a batch process and as described in, for example, U.S. Pat. No. 5,319,053, the disclosure of which is herein incorporated by reference. The continuous cascade process gives the expected small variation in the allophanate oligomer distribution, however, these products are interchangeable with the products of a batch process.